stock or fmf pipe to break in new top end?

[tank_tech]

i am about ready to break in my top end rebuild. should i jet it back to stock and use the stock pipe to break it in, or should i leave the jetting(which im not sure is correct) and the fmf fatty and jst break it in using that?

 

[whenfoxforks-ruled]

Put the pipe you want on, jet it, done. The stock jetting correction table should be fine for a start. Even if its lean, will not kill the engine for jetting! You correct it and continue. Till its broke in there is no need but to get the pilot and needle working crisp. Then check wfo when it clears out and the rings are seated. The pipe and silencer just swing the power delivery around and some slightly effect jetting. Altitude, temperature and barometric pressure influence your engine more than a bolt on, usually. There are some pipes, the sst for 1 on some YZ125's, can alter it enough to need higher octane fuel to prevent detonation though. Vintage Bob